1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, for feedback-controlling an output value of an air-fuel ratio sensor that is disposed at a location midway or downstream of a catalyst in an exhaust passage of the engine such that the output value becomes equal to a target value.
2. Description of the Related Art
Conventionally, there has been known an air-fuel ratio control system for an internal combustion engine, which is disclosed e.g. in Japanese Patent No. 3961275. The engine has an exhaust passage provided with an exhaust purification device for purifying exhaust gases and an air-fuel ratio sensor disposed at a location upstream of the exhaust purification device. The air-fuel ratio sensor is of a type which linearly detects an air-fuel ratio of an air-fuel mixture based on the concentration of oxygen in exhaust gases.
This air-fuel ratio control system calculates a basic fuel injection amount and sets a target air-fuel ratio, according to operating conditions of the engine, and calculates, when conditions for performing feedback control are satisfied, a feedback correction coefficient by the feedback control such that the air-fuel ratio detected by the air-fuel ratio sensor becomes equal to the target air-fuel ratio. Then, a final fuel injection amount is calculated by multiplying the above-mentioned basic fuel injection amount by the target air-fuel ratio and the feedback correction coefficient, and the air-fuel ratio is controlled using the final fuel injection amount.
Further, when resuming the feedback control after interruption thereof, the air-fuel ratio control system sets the integral term of the feedback control to a predetermined initial value set in advance according to the operating conditions of the engine, and holds the same at the initial value for a predetermined time period thereafter, whereby the controlled variable of the feedback control is properly calculated when resuming the feedback control.
In the above-described conventional air-fuel ratio control system, however, at the resumption of the feedback control and for the predetermined time period thereafter, the integral term of the feedback control is always set to the predetermined initial value dependent on the operating conditions of the engine. No learning results of the integral term calculated according to the injection characteristics of the fuel injection valve during the preceding execution of the feedback control are reflected on the initial value at all, and hence there is a fear that the feedback control is started in a state where the integral term is quite different from an appropriate value. In this case, there occurs a delay before the integral term reaches an appropriate value, and hence it is impossible to properly calculate the controlled variable of the feedback control during the time period, which results in the degraded accuracy of the air-fuel ratio control.